1. Field of the Invention
The present invention relates to the field of the fabrication of semiconductor devices, and, more particularly, to a method of manufacturing thinned die for flip-chip applications by employing a substrate thinning process to reduce the thickness of substrates bearing, for instance, a plurality of die for forming integrated circuits.
2. Description of the Related Art
There is an ongoing trend in the semiconductor industry to increase the size of substrates employed in the manufacturing process. Currently, 400 mm substrates are intended for industrial application for large scale production technologies, for example, for memory or microprocessor devices. To ensure safe and readily practicable substrate handling during the entire manufacturing process, the thickness of the larger substrates is also increased. Thus, 400 mm substrates typically have a thickness of approximately 800 μm, whereas, for example, 200 mm substrates typically have a thickness of approximately 725 μm. On the other hand, the number of applications requiring thinned die is growing, for example, smart card, multi-chip-module, stacked die module and high power module applications, wherein, additionally, a tendency for a reduced target thickness of the die for the different applications is discernible. For smart card applications, for example, a desired target thickness of 20 μm is on the horizon.
Usage of thinned die offers a plurality of advantages. Ultra-thin die, e.g., 20 μm or less thick, exhibit a high flexibility so that they appear to be appropriate for future smart card applications having a desired total thickness of 250 μm or less. Due to the high flexibility, ultra-thin chips are able to withstand the mechanical stress exerted from carrying and using in everyday life. For smart card applications, typically, thinned die having a thickness in the range of 20-200 μm may be utilized. Further, ultra-thin die may be employed in stacked die packages to achieve a higher number of transistors per unit volume of a module. Such high transistor density modules are appropriate for low power applications, for example, in stacked memories. For high power applications, heat removal in stacked die packages is not sufficient. For stacked memory applications, die having a typical thickness in the range of approximately 40-100 μm are used. For high power applications, the employment of thinned die is also advantageous since heat removal from the backside of a chip may be improved due to the reduced heat resistance of thinned and polished chips. In high power applications, the thinned die typically have a thickness in the range of approximately 50-400 μm.
In addition to the improvement of the thermal behavior of thinned chips, the electrical performance of an integrated circuit on a thinned chip may be improved. In particular, in silicon-on-insulator (SOI) devices, the electrical performance gain may be more relevant than the improvement of the thermal behavior, since, in SOI devices, the thermal resistance of a backside contact is increased compared to a bulk substrate device, due to the pure thermal conductivity of the insulator layer.
Thin die are also employed for wire bonding and flip-chip bonding. Substrate thinning, however, is a crucial step in the packaging related processes. In particular, grinding of substrates provided with solder bumps is a major concern in manufacturing thin die for flip-chip bond applications since, during backside grinding, the bumps disposed on the front side are also subjected to mechanical load and consequently may be damaged. To illustrate the challenge of grinding of bumped substrates 100, a substrate 102 with solder bumps 106 thereon is depicted in FIG. 1a. A substrate 102 may bear a plurality of die 120 comprising an integrated circuit 118. The integrated circuit 118 is connected to several bond pads 114. Between the bond pads 114, a polyimide passivation layer 116 is formed on the substrate surface. On the bond pads 114 and on parts of the polyimide passivation layer 116, an under bump metallization (UBM) layer 112a is disposed. On the UBM layer 112a, a solder bump 106 is formed over the bond pads 114. The solder bump 106 may comprise a base portion 110 and a solder portion 108. Bump protection tape 104 may be laminated on the bumped substrate surface to protect the substrate 102 during processing of the backside of the substrate.
The die generally may be thinned at the substrate level or at the die level, i.e., prior to or after dicing. Thinning on the substrate level allows processing of the plurality of die of an entire substrate 102 by a common process. Consequently, thinning at the substrate level achieves a low total thickness variation (TTV). Furthermore, thinning at the substrate level provides the throughput required for large scale production. Although, in the so-called “dicing before grinding” (DBG) process the substrate 102 is subjected to a dicing step before grinding, the method pertains to the grinding at substrate level category since the cutting is performed only to a certain depth somewhat deeper than the final desired die thickness so that the grinding step is performed mainly on the entire backside of the substrate 102 until the substrate 102 is separated into individual die at the end of the thinning process. In that phase, the individual die may be held in position by a protection tape 104 adhered to the substrate/die front side. After the grinding process, the die may be placed in a die frame before removing the front side protection tape 104.
Thinning at the die level, on the other hand, reduces the risk of substrate breakage (related to thinned substrate handling) and of die defects caused by chipping in the dicing process since the region of the die (edge of the backside) that is typically affected by chipping may be removed in the subsequent thinning process.
The substrate thinning process may be performed by any method appropriate for removing the substrate material, for example, mechanical methods, chemical methods or combinations thereof, such as coarse/fine grinding, dry polishing, etching or chemical mechanical polishing (CMP). In general, different methods are combined to provide the thinned die with the desired backside surface quality by a cost- and time-effective process. The substrate thinning processes, in particular, substrate grinding processes, are well known in the semiconductor industry and, thus, are herein not described in detail. An overview of characteristics of the employed substrate thinning methods, however, is provided in the following to illustrate the present application.
Typically, substrate grinding is performed by subjecting a rotating substrate 102 to a treatment by means of rotating cup wheel grinding tools. Coarse grinding using a cup wheel tool with a coarse grid may be utilized to remove most of the amount to be removed from the substrate 102 (such as 90% or more). This process, however, leaves a rather rough backside surface and causes severe crystal damage in a region close to the ground backside surface(sub-surface) reaching to a depth of approximately 5-30 μm (depending on the characteristic of the grinding tools used). Thus, typically, fine grinding using a cup wheel tool with a fine grid is subsequently performed to reduce the backside surface roughness and the sub-surface crystal damage. Since the fine-grind step is based on the same principle, there is still crystal damage present in the backside sub-surface (extending approximately 1-10 μm). The sub-surface damage is related to a plurality of tiny cracks created in the grinding process. These tiny cracks, however, introduce stress and dramatically reduce the stability of the substrate 102 and heighten the bowing of the substrate 102. Thus, to improve the strength of thinned die, a stress relief step removing or at least reducing the sub-surface damage may be performed subsequently. Dry polishing, chemical mechanical polishing (CMP), dry (plasma) etching and wet etching may be employed for stress reduction by removing the sub-surface damage. Wet etching may be performed rather by a spin etch process than a bath etch process to achieve a better final thickness uniformity. Acidic compounds containing HF and HNO3 may be applied to the damaged region. Advantageously, an etching process using HF provides a clean surface so that, compared to mechanical polishing operations, an additional cleaning step may be avoided.
Due to the fact that etching exerts, contrary to grinding, substantially no mechanical stress on the substrate 102 during the removal process, etching is the favorite process for forming ultra-thin die to reduce the risk of breakage during the thinning process. Despite this, there is no strict lower limit for mechanical thinning of substrates 102 so that, for example, well-adapted grinding processes have been employed to achieve a final thickness of the die of 20 μm. Since, furthermore, grinding methods, in general, exhibit a higher removal rate, grinding is widely used for substrate thinning.
Die thinning is performed on completely processed semiconductor substrates 102, wherein approximately 500 or more process steps may be performed prior to the thinning step. Thus, any severe substrate damage, e.g., breakage of the substrate 102, involves substantial costs. Thus, the choice of an appropriate thinning process is important for the cost-effectiveness of the die thinning process. On the other hand, since the risk of damage for thinned substrates 102 during handling and processing is high, the cost-effectiveness of the thinning process is further determined by the point of integration into the conventional semiconductor manufacturing process. To provide an overview of the possible points of integration, a typical process sequence of a solder bumping process, comprising the steps of forming a polyimide passivation layer 116, forming solder bumps 106, reflowing the bump material, performing a wafer-level functional test, dicing and assembling, is described in the following.
Completely processed semiconductor substrates 102 when leaving the wafer facility are provided with a passivation layer, e.g., silicon oxide or silicon nitride, and exposed (aluminum, gold, copper) bond pads 114 to electrically connect the devices, e.g., by wire bonding. To provide the die with bumps 106, required for flip-chip bonding, the processed substrate 102 is, as a first step, subjected to a further passivation process to form a polyimide layer 116 on the substrate 102 exposing the bond pads 114.
Polyimide materials exhibit a good thermal stability (<450° C.), a low coefficient of thermal expansion, excellent dielectric properties, mechanical toughness and chemical resistance. Thus, the polyimide passivation layer 116 may serve as a buffer for mechanical stress and a protection layer. Polyimide layers, for example, may be utilized to shield from a particles which may be radiated from the solder bump material. While the required thickness of the polyimide layer for α particle protection is approximately 40 μm, for a stress buffer functionality, a thickness of 4-6 μm is sufficient. Polyimide layers may further be employed as insulation layers in bond pad redistribution applications, wherein a peripheral bond pad arrangement may be redistributed to, for example, an area array distribution to allow higher chip I/O counts or reduce the bump pitch requirements.
Typically, the substrate surface is cleaned and pre-processed by a primer treatment to improve the adhesion of the polyimide layer. Some polyimides have built-in adhesion promoters to eliminate an additional primer treatment step. The polyimide layer is typically spin-coated (possibly in a two-step process, to achieve the required thickness) on the substrate 102 to assure a good uniformity, and subsequently cured, typically in a multi-step curing process. Hot plates are commonly used for an initial bake after application in a temperature range of approximately 50-150° C. Final curing is usually performed in a furnace or programmable oven in a temperature range of approximately 280-350° C.
Polyimide layers are commonly patterned by wet etch processes, dry etch processes, laser ablation or by the usage of a photo-definable polyimide. Photo-definable polyimides reduce the number of process steps required to form a polyimide pattern and allow patterning of relatively fine features. Dry etch techniques are typically employed to form very fine features with high aspect ratios, whereas wet etch processing is typically used to pattern coarse features. For a currently achieved bond pad size of approximately 100 μm, the wet etch process is still applicable. The spin-coated polyimide layer is partially cured and a positive resist is deposited thereon, baked, imaged and developed. The developer simultaneously wet etches the underlying polyimide layer in the imaged regions. After a water rinse, the resist is stripped using a liquid resist stripper. The patterned polyimide is then completely cured to accomplish the imidization process and to remove residual solvents.
Patterned polyimide layers may also be formed by screen or stencil print techniques requiring only a print and a cure step to form a patterned polyimide passivation layer 116. These print techniques allow deposition of polyimide layers of a thickness of more than 40 μm in a single print step but are restricted to coarse patterns with bond pad openings greater than about 125 μm. For smaller feature sizes, the entire wafer may be coated by screen printing and, after cure, the bond pad lands may be reopened by using laser ablation techniques.
Independent of the chosen polyimide passivation process, polyimide passivation 116 requires a lot of substrate handling steps which are, in general, carried out by substrate-handling robots integrated into the polyimide line.
After the formation of the polyimide passivation layer 116, the substrates 102 are transferred from the polyimide line to the bumping facility. The solder bumps 106 may be formed by electro/electroless plating, screen printing or an evaporative bump process. The evaporative bump process is a well-established, widely-used solder bump process, based on evaporation of metal through a mask to form the bumps 106 over the bond pads 114 of the die. In the evaporative bump process, a so-called shadow mask having openings with tapered sidewalls is clamped onto the substrate 102 so that the mask openings are aligned to the bond pads 114 to be bumped. The taper of the shadow mask, which is typically formed of molybdenum, facilitates the removal of the mask after solder bump 106 formation.
After mask alignment, an argon sputter etch is employed to remove die bond pad oxides and to ensure low electrical contact resistance. Subsequently, an under-bump metallization (UBM) layer 112a comprising chrome/chrome-copper/copper/gold sub-layers is deposited through the shadow mask on the exposed regions of the substrate 102, i.e., on portions of the bond pads 114 and on the sidewalls of the polyimide passivation layer 116. This layer acts as an adhesion layer, provides an electrically conductive diffusion barrier and establishes a good mechanical base 110 for the solder bump 106. The next deposition step of the evaporative bump process is to evaporate lead, followed by tin, to form the bulk of the bumps 106. After the final deposition step, the shadow mask is removed. In the next step, the bumps 106 are reflowed to homogenize the lead/tin solder and to allow the tin to form an intermetallic compound, for example, with a copper sub-layer of the UBM layer 112a to provide the required adhesion between the die and the bumps 106. The above-described bumping process requires a lot of substrate handling steps which may also be carried out by robots.
In a final inspection step, the reflowed bumps 106 are inspected to detect defective bump structures. The inspection process may be performed in the bumping section or the substrate may be transferred to a separate inspection station. Typically, substrate-handling robots are employed to handle the substrates 102.
The bumped substrate 102 is subsequently transferred to a test section and subjected to a functional test to detect defective die and to characterize the performance of the die formed on the substrate 102. The die may be tagged corresponding to the test results or the test results for each individual die of the substrate 102 may be transferred to the assembly section so that the die may be sorted according to the results of the test. The test is typically performed by means of auto-prober tools including wafer-handling robots.
After testing, the substrate 102 may be again transferred to the reflow section to heal damage caused by the pins of the probe cards. Prior to the reflow step, the substrate 102 may be subjected to a cleaning process. Performing the bump reflow completes the manufacturing process on substrate level and the substrate 102 is transferred to the assembly section.
The substrate 102 is diced to form individual die which may be sorted, for example, according to the result of the above-described substrate-level functional test. The resulting individual bare die are ready to be utilized in any flip-chip application not requiring thinned die.
To produce thinned die, however, a substrate thinning process is integrated into the above-described bumping process sequence. In principle, the thinning process may be included at any bumping process step, i.e., before or after each individual process step of the polyimide passivation process, solder bumping process, reflow process, wafer-level functional test, or dicing since the substrate handling/transferring equipment may be adapted for handling of thinned substrates 102. Handling/transferring of thinned substrates 102, however, even with adapted equipment, increases the risk of breakage of the substrate 102. Additionally, the adaptation of the equipment increases the manufacturing costs. Therefrom, it is suggested to perform the thinning process after dicing to avoid handling/transferring of thinned substrates 102. Thinning of individual die, however, is a cost- and time-consuming operation that does, in addition, not meet the desired thickness uniformity requirements. Furthermore, the substrate thinning process impairs the solder bumps 106, in particular when a grinding method is employed, even when the bumped side is laminated by a conventional bump protection tape 104. For some applications, depending on the employed solder bump material, for example, for microprocessor applications comprising typically solder bump materials with high lead content (greater than 90%), the damage is, in general, unacceptably high and leads to high yield loss. Deposition of an additional protection layer to entirely embed the formed solder bumps 106, for example, by means of spin-coat deposition, is time- and cost-consuming and bears the risk of adversely affecting the bumped substrate 102, for example, by inducing thermal stress or by contamination. As a consequence, the thinning process is, according to the prior art, performed on substrate-level prior to the formation of the solder bumps 106 as illustrated by the process sequence 150 of FIG. 1b. Consequently, backside grinding is performed before or after the polyimide application, although the high risk of breakage of the thinned substrates 102 during the huge number of processing and handling steps, and the demand for special handling/transferring equipment for thinned substrates 102 in the bumping, reflowing, testing and dicing processes, reduces the cost-effectiveness of the process.
In view of the above-mentioned problems, there exists a need for an improved process sequence for thinning of substrates bearing die for flip-chip mounting.